Mr Jack Greenwood
|PGR Student in the Department of Physics||144b|
|Postgraduate Student in the Centre for Materials Physics||144b|
After achieving a first class degree in Physics (MPhys, Hons) from the University of Durham in 2016 and gaining experience working in the European Fusion Reference Laboratory here at Durham, I am working on an experimental PhD under the supervision of Professors Damian Hampshire and Ray Sharples. I am a member of the Department’s Superconductivity Group and the EPSRC Centre for Doctoral Training in the Science and Technology of Fusion Energy, based in York (http://www.fusion-cdt.ac.uk/). The Fusion CDT is an institution that pulls together world class expertise in the field of nuclear fusion, with an emphasis on research into designing fusion reactors for research and eventually, power generation. The CDT is a collaboration between 5 major UK universities: The University of York, University of Durham, University of Oxford, University of Liverpool and University of Manchester, as well as other key partners in the fusion sector such as the Culham Centre for Fusion Energy (CCFE), at which the Joint European Torus (JET) is housed.
High temperature superconductors have the potential to replace traditional low temperature superconductors in future applications such as magnetic confinement fusion (MCF), magnetic resonance imaging (MRI), particle accelerators, magnetic levitation and power transmission. They have much higher values of upper critical magnetic field (Bc2), critical temperature (TC) and critical current density (JC). Many commercial high temperature superconductors are fabricated as ‘coated conductors’ which contain a ~micron thick layer of superconducting (RE)BaxCuyOz, with (RE) being a rare earth element such as Yttrium or Gadolinium (see right).
In high field magnet systems such as those used for MCF, a superconductor is subjected to large Lorentz forces, due to the high magnetic fields and large currents that are present. If coated conductors are used, the Lorentz forces induce strains in the superconducting (RE)BaxCuyOz layer and the presence of strain (ε) causes the superconductor’s values of Bc2, TC and JC to change. The main focus of my work is to determine the mechanisms responsible for the strain dependence of JC in coated conductors, which will allow manufacturers to optimise their fabrication processes and engineers to improve their magnet designs. The scope of the work is summarised below.
Experimental Apparatus & Facilities
Bending beams (springboards) are used to apply both compressive and tensile uniaxial strains to (RE)BaxCuyOz coated conductors along the direction of current flow (see below, left) . We can apply strains ranging from -1.2% ≤ εxx ≤ 0.6%. The bending beam is attached to a bespoke probe which can be used in our world class, 15 T horizontal magnet system. Using the horizontal magnet system, we can investigate the angular dependence of JC. Critical currents are measured using a standard 4-terminal technique. We have also developed a variable temperature cup which can be used to measure JC for temperatures ranging from 4.2 K up to TC (~90 K).
Recently we developed a biaxial sample holder known as a ‘Crossboard’ (see below, right) which can be used to apply in-plane biaxial strains (along the x-and y-directions) to a (RE)BaxCuyOz coated conductor . Some early results for the in-plane biaxial strain dependence of the critical current density (JC(εxx, εyy)) are discussed below.
It is well established that the uniaxial strain dependence of the critical current density JC(εx) is either parabolic or linear,depending on the coated conductor’s fabrication method (e.g. see bottom left). For coated conductors with parabolic relationships for JC(εxx), the peak in JC(εxx) may occur at a non-zero value of εxx. The standard explanation for the location of the peak is that it occurs at the applied strain where the (RE)BaxCuyOz layer has zero net uniaxial strain . The position of the peak is influenced by the thermal strains that the (RE)BaxCuyOz layer is subjected to because of temperature changes during fabrication and cool-down. This is because the different layers within the coated conductors have different coefficients of thermal expansion.
However, using the crossboard, we have shown that if we fix the x-strains to have the same values as the y-strains (i.e. εxx = εyy) on a tape with a parabolic uniaxial strain dependence, we obtain a linear relationship for the strain dependence of JC(εxx = εyy) (see bototm right). We have explained this novel behaviour by considering the (RE)BaxCuyOz layer as a 1D chain of (RE)BaxCuyOz single crystals that have their a- or b-axes aligned with the length of the tape. Literature shows that the strain dependence of TC in single crystals of (RE)BCO is anisotropic with respect to the crystal axis that the strain is applied along. We have related the anisotropic strain sensitivity of TC of (RE)BaxCuyOz single crystals to the macroscopic critical current density JC .
- Designing and commissioning a bespoke probe that can be used to investigate the biaxial strain dependence of JC in our 15 T horizontal magnet system, as well as a new biaxial sample holder which can strain a (RE)BaxCuyOz coated conductor in-situ.
- Relating the biaxial strain dependence of JC to strain measurements on single crystals of (RE)BaxCuyOz.
- Investigating the JC’s of (RE)BaxCuyOz coated conductors with advanced pinning centres- as a function of magnetic field, magnetic field angle, temperature and uniaxial strain.
 Super Power Inc., url: http://www.superpower-inc.com/content/2g-hts-wire Accessed 22/10/18.
Sunwong, P., Higgins, J.S. & Hampshire, D.P. (2014). Probes for investigating the effect of magnetic field, field orientation, temperature and strain on the critical current density of anisotropic high-temperature superconducting tapes in a split-pair 15 T horizontal magnet. Review of Scientific Instruments 85(6): 065111.
 Greenwood, J.R, Surrey, E. and Hampshire, D.P. Biaxial Strain Measurements of JC on a (RE)BCO Coated Conductor. IEEE Transactions on Applied Superconductivity, scheduled for June 2018 (vol. 28, issue 4).
 Osamura, K., Machiya, S. & Hampshire, D. P. (2016). Mechanism for the uniaxial strain dependence of the critical current in practical REBCO tapes. Superconductor Science and Technology 29(6): 065019.
 Sunwong, P., Higgins, J.S., Tsui, Y., Raine, M.J. & Hampshire, D.P. (2013). The critical current density of grain boundary channels in polycrystalline HTS and LTS superconductors in magnetic fields. Superconductor Science and Technology 26(9): 095006.
 Greenwood, J.R, Surrey, E. and Hampshire, D.P. The Biaxial Strain Dependence of JC on a (RE)BCO Coated Conductor at 77 K in Low Fields, IEEE Transactions on Applied Superconductivity, scheduled for 2019.
- Applied Superconductivity Conference 2018, 28th October - 2nd November 2018, Seattle, Washington, USA;
- 13th European Conference on Applied Superconductivity, 17th - 21st September 2017, Geneva, Switzerland;
- Culham PhD Showcase, 11th - 12th July 2017, Oxford, UK;
- Joint CDT Nuclear Energy Event, 24th May 2017, York, UK;
- Fusion Frontiers and Interfaces Workshop, 8th - 10th May 2017, York, UK.
Fusion Sandpit Conference 2018, 30th – 31st August 2018, Fusion CDT, York, UK;
Fusion Frontiers and Interfaces Workshop 2018, 30th April – 4th May 2018, York, UK;
Applied Superconductivity for Fusion Technology, 21st March 2017, Oxford, UK.
- Greenwood, Jack R., Surrey, Elizabeth & Hampshire, Damian P. (2019). The Biaxial Strain Dependence of Jc of a (RE)BCO Coated Conductor at 77 K in Low Fields. IEEE Transactions on Applied Superconductivity 29(5): 8637756.
- Greenwood, Jack R., Surrey, Elizabeth & Hampshire, Damian P. (2018). Biaxial Strain Measurements of Jc on a (RE)BCO Coated Conductor. IEEE Transactions on Applied Superconductivity 28(4): 8400705, 1.